Solution
Apparently the Microsoft.SqlServer.Dts.Design dll belonging to SSDT 2012 is not installed in the GAC during installation. The version belonging to SSDT 2010 is still in the GAC. On the left side the assembly before fixing this bug and on the right side the assembly after fixing this bug. Ironically the assembly from 2012 has a lower version than 2010. Dave found a solution for it.

Microsoft.SqlServer.Dts.Design dll before and after fixing the bug.

1) Close SSDT 2012
Close Visual Studio (SSDT) 2012.

2) Visual Studio Command Prompt
Open the Visual Studio Command Prompt as administrator (otherwise the gacutil will return an error). You can do this by right clicking the shortcut and then choose Run as Administrator.

4) Run GACUTIL
Run GACUTIL with the following parameters to add the assembly from SSDT 2012 to the GAC:gacutil/if Microsoft.SqlServer.Dts.Design.dll
Parameter /if will force the installation of this assembly regardsless of any existing versions of the assembly

gacutil/if Microsoft.SqlServer.Dts.Design.dll

5) Open SSDT 2012
Now open SSDT 2012 and try running the package again to check the fix.

*UPDATE (see comment Alejandro Bello)*
If you didn't install SSDT 2010 then the GAC Util is located in a different folder. Start the regular Command Prompt as Administrator and then execute the following commands:

Sunday, 5 October 2014

Had a nice day at SQL Saturday #336 in Utrecht! The PowerPoint slides of my SSIS Development Best Practices session are available for download.
I added some screens, text and URL's for additional information (see notes in PowerPoint)